Resilient retainer rings having spaced apart lugs are widely used in the assembly of articles of manufacture to hold various components thereof together. For example, the wrist pin joining the piston and connecting rod of an internal combustion engine is held in the piston bore by an internal retainer ring inserted into grooves in opposite pilot sections of the piston bore. Retainer rings having a flat side profile and shape shown in FIG. 11 have in the past been used in holding the wrist pin in the piston bore of automobile diesel engines. Various attempts have been made by prior art workers to devise automated or semi-automated apparatus for compressing these flat retainer rings and maintaining the ring at the compressed assembly size for insertion in the piston bore. These apparatus have generally utilized an external containment system such as a thin tapered sleeve positioned around the compressed ring to maintain the compressed condition at the assembly location. However, the use of such thin external containment sleeves has been disadvantageous as a result of the vulnerability of the sleeves to wear by friction contact with the ring and also as a result of the requirement for additional clearance space necessary to enable insertion of the retainer ring and external sleeve in the piston bore.
Illustrative of this and other prior art approaches are the Erdmann U.S. Pat. Nos. 2,840,892; 2,870,529 and 3,134,168.
Due to the inadequacies in existing ring compressing apparatus, flat profile retainer rings in many cases are still assembled in the bore of certain pistons by hand using plier type tools with probe pins to engage the small hole provided in each lug of the retainer rings. Of course, assembly in this manner is more time consuming than assembly by automated or semi-automated means and also is more inconsistent in terms of the achievement of uniform compression of the retainer rings and accurate placement in the piston bore.
Thus, what is still needed is an apparatus, preferably amenable to automatic or semi-automatic operation, for compressing such retainer rings to assembly size and maintaining the compressed ring size with the external ring periphery fully exposed for insertion into the piston bore or other part internal bore.